The present invention concerns to lead- and arsenic-free optical lanthanum flint glasses characterized by their optical position with a refraction power of 1.73≦nd≦1.82 and an Abbé value of 43≦Vd≦53 as well as their use.
The development of the market in the field of the so called “optical data transfer” trends increasingly to smaller devices which are still effective and which could consequently achieve more and more higher data transfer rates. This trend is also finding in the application fields imaging, digital projection, photo lithography, laser technology, wafer/chip technology as well as for the telecommunication, optical communication engineering and optical/illumination in the automotive sector.
Further, in all sectors of this application field the traditional “read only technology” is more and more displaced by the “read & write technologies”. Therefore, the requirements on the optical systems and thus on the optical material are changing.
While the “read only technologies” can be operated based on the separation in the two spatiotemporal strictly separated operating mode (writing and respectively reading) in the monochromatic mode and therefore the writing process could be carried out with light of the same wave length as well as the reading process, which could only carried out in the later operation, this is not possible for the “read & write technologies”. At this juncture, the wave length of the “writing jet” has to diverge from the wave length of the “laser jet” about at least 2-5 nm to lower values. Otherwise, not both modes in one optical head could be operated in the device. If a writing head and a reading head are necessary, this means that two different heads have to be combined in one device, and the technical effort as well as further the size and at least the costs of such devices would be unacceptable.
The wavelength differentiation results from the necessity to separate the reading- and writing jet in the optical system to exclude important aberration by interference- and low light level effects. The lower the difference of both wavelengths can be kept while maintaining the necessary total separation, the easier such an optical system is realize. In this case, the term “easy” concerns to the amount of the necessary optical components and therefore both, the minimal overall size of the module as well as the costs.
However, the minimal wavelength difference as may be necessary for the complete division depends on the dispersion of the glass components in the optical system. The higher the dispersion (and therefore the lower the Abbé coefficient), the further the both respective monochromatic jets are fan out and respectively broadened, till they finally interfere, which is contradictory to the aspired division. In reverse this means for glasses with this application intention: With diminishing dispersion, only low wavelength differences may be converted, which correspond to the aspired intention of a cheap product.
Besides this improvement, a further improvement results from the low dispersion: It is possible to work with clearly lower wavelengths based on in principle identical wavelength differences in absolute terms. In general, the dispersion has a higher effect to rays with declining wavelengths.
In addition to the disadvantage of the larger minimal wavelength difference of conventional glasses with a higher dispersion, also an undesirable increased absolute wavelength, as compared to low dispersive glass types, follows. A low absolute wavelength region of operation is favoured again in view of the application of the complete systems: The lower the operating wavelengths, the higher the achievable information packing density (relating to the unit of area of the data carrier material) are formed. In addition to the maximized information density, the access time is also optimized by shorter ways, namely reduced.
Also, the refraction power position has an important influence to the practical property of such a complete system: The actual “Pick up lenses” are defining with their refraction on power, both, the absolute wavelength region of operation of the combined write-read jets as well as the focal length of the system. As to the focal length, the interrelationship is as follows: The lower the focal length of such a system, the lower is there geometric dimension, which results directly in the component size and therefore massive in weight and costs. Therefore, a higher refraction power in the important wavelength range is desired.
Besides, this is comparable for optical components of all above mentioned application areas. A further advantage of a high refraction power is the possibility to coat “pick up lenses” aspherical: The lower the transmission property of the glass at the operating wavelength, the poorer is the yield of the light in the system. The light intensity has a direct influence to the writing-reading quality of the systems. The poorer the yield of the light, the higher must be the efficiency of the light source, and due to this there are additional refrigerating sets necessary, resulting in costs and expenditure of unacceptable ranges.
It is known, that high transmissions in the wavelengths working range of all optical systems are of particular importance. The lower the transmissivity of the glasses are at the working wavelengths, the more worst is the yield of the light of the system. However, the illuminance goes directly in the write-read quality of the systems. The more worst the illuminance, the higher the efficiency of the light source has to be, whereby the additional cooling units are required again, following in costs and expense in an unacceptable range.
Additional to the optical values, also physical and chemical parameters are specified for glasses for the use in the above described application. These parameters are the low specific density and the good coating property, whereas these both conditions are leading to a restriction of the composition of the glasses to defined components.
The specific density of the optical materials of these systems is very important. The “pick up lenses” as a component of the combined write-read head is movable elements of the system. The heads are moving for the actual data transfer above the data carrier. Therefore, the access times and the track densities are depending on the possibility of the fast and exact positioning of the heads. Thus, the higher the specific density of the glass building components, the higher is the mass of the mobile unit, which is then more inert and therefore slower in positioning. Due to this, the specific density of the glasses according to the present invention should be low.
For many application areas, the reduction of the mobile mass of the glasses according to the present invention is very important. The “handiness” of lens systems, for example for photography, the projection and in future for the glass fiber technology and glass building component technology (e.g. for the use in the area of mobile optical computer like for example optical lap tops), is a important criteria.
The aspherical coating of the Pick-Up lenses is organic chemical nature, like the common coatings of optical lenses and prisms. To obtain an adequate bonding of the optical layer on the basic glass, the glass material should contain components, which allow a strong bond to organic materials.
Regarding to the aspect of materials procession concerning molten mass/hot forming, there is an increasing requirement for so called “short” glasses, whereby these are glasses whose viscosity are variegating strong depending on temperature. This attitude shows the advantage during the process that the times for the hot forming; the “form fit times” could be shortened. Therefore, the throughput is increased and at the same time the forming material is treated with care, which has an extremely positive effect to the total production costs. Also, glasses with a stronger crystallisation gradient could be better processed since due to higher throughput a faster cooling down is possible. Therefore, problems are avoided, which occurs on “longer” glasses, like pre germination with following difficulties at the secondary hot forming.